This invention relates to corrosion control in steam condensate systems and other aqueous systems in which the mineral content is relatively low.
More particularly, this invention relates to the use of methoxypropylamine in combination with hydrazine to control corrosion in steam condensate systems or in other low solids aqueous systems.
Condensate corrosion protection is becoming an increasingly important aspect of plant operation. In these energy-concious times, an increase in the quantity and quality of condensate will result in water and heat savings for the total boiler system.
Historically, the action of dissolved gases such as oxygen and carbon dioxide have been two of the main factors that lead to condensate corrosion. In order to understand the role of oxygen and carbon dioxide in corrosion, one must understand the electrochemical nature of corrosion. Pure water has very little effect on pure iron, but this situation is seldom encountered. Under most conditions, there is a tendency for iron to dissolve in water, and two electrons are released for each atom that dissolves. These electrons are transferred to hydrogen ions present in water, and the ions are reduced to elemental gaseous hydrogen. All action ceases at this point if the hydrogen remains on the surface of the metal since a protective coating is formed that interfers with the passage of electrons. However, any agent which increases the number of hydrogen ions present in the water, or which will cause the removal of the protective film, serves to increase the rate of corrosion.
When carbon dioxide dissolves, it reacts with water to form carbonic acid, which supplies additional active hydrogen to the system. Iron displaces the hydrogen from this acid. When oxygen is also present in the water, a two-fold reaction takes place. Some molecules of oxygen combine with the displaced hydrogen and thus exposes the metal to fresh attack. Other oxygen molecules combine with iron ions to form insoluble rust compounds.
A greater corroding influence than the mere dissolving tendency of iron is the existence of a heterogeneous surface in commercial iron and steel due to the presence of surface imperfections which tend to form couples with the adjacent base metal. Electrons are released from the anodes of these couples to the hydrogen ions at the adjacent cathodic surface, thus increasing the corroding area and accelerating the rate of corrosion.
The first product of corrosion may be converted of ferric oxide, which is only loosely adherent and aggravates corrosion by blocking off areas to oxygen access. These areas become anodic and iron oxide couples are set up. The iron under the oxide deposit then dissolves, and pitting develops. Carbon dioxide attack results in thinning or grooving of the metal.
For those systems that will permit it, filming amines will give condensate corrosion protection against both oxygen and carbon dioxide. However, many industrial systems cannot tolerate filming amines and must use neutralizing amines.
The ideal neutralizing amine should have the following characteristics:
1. The distribution ratio should be high enough so that a considerable amount of the neutralizing amine fed to the boiler will end up in the condensate. This will reduce the loss of neutralizing amine via blowdown.
2. The distribution ratio should not be too high in order to keep losses due to aeration and venting to a minimum. The distribution ratio is the ratio of the amount of amine in the vapor phase to that in the liquid phase.
3. The basicity value should be moderately high or very high so that the amine will efficiently neutralize all carbon dioxide that it encounters.
4. The neutralizing amine should have sufficient hydrolytic-thermal stability so that it will not break down to ammonia and other compounds in the boiler or in superheated or saturated steam.
5. The neutralizing amine should be a water-soluble liquid for feeding convenience.
Neutralizing amines such as cyclohexylamine and morpholine have been used but they have several disadvantages. For example, cyclohexylamine has a high distribution ratio and accordingly, substantial cyclohexylamine escapes the system through the deaerator vent. Morpholine, on the other hand, has a low basicity value which means that more morpholine is required to attain high pH's in the condensate system and it also has a very low distribution ratio which means that substantial amounts are lost via blowdown.